they r to comletley different things, with different purposes.
prime: priming parts is painting it with a primer, (a special type of paint, kinda like thinned putty)which servers serveral purposes. it will provide a solid base coat colour for subsequent colour coats, aswell as aiding with adheision of the the paint. it will help u spot surface imperfections, and if any puttying was done, it will provide a uniform surface. it will fill minor scrathes and rough areas. i recewntly have been using mrbase white 1000 as my primer and am very happy with the results.
now, a was is a very diluted paint which u apply to a model to higlight certain thing, such as bolts, panel lines etc... becauswe it is so thin it will run easily where u want it to go. i feel tha enamels are best for wshes.
hope this helps ya out mate!